Remove tray icon and related methods
"gtk_status_icon_*" is deprecated, the replacement proposed by GTK is GNotification, which does a completely different thing.
A tray icon IMO is not really needed. There is still the extra-window for the copy operation, which should be sufficient. (Other file managers as well do not show a tray icon)
And the deprecated "gtk_status_icon_new_from_icon_name" seems to cause scaling issues. (See Issue #495 (closed))
Fixes #495 (closed)